The present invention relates to a method for compressing atmospheric air using a centrifugal or axial compressor, this air being at a natural temperature between xe2x88x9220xc2x0 C. and +5xc2x0 C. approximately, outside of periods of freezing fog, supercooled rain or snow, of the type in which the droplets contained in the air are possibly separated, then the air is subjected to a filtration stage or to several successive filtration stages.
The air thus compressed may, for example, be intended to supply an air distillation apparatus or a gas turbine of the industrial type. The term xe2x80x9cgas turbine of the industrial typexe2x80x9d is to be understood as meaning gas turbines of the so-called xe2x80x9cheavy dutyxe2x80x9d type, as opposed to so-called xe2x80x9caeroengine-derivedxe2x80x9d turbines.
Atmospheric-air compressors in air distillation installations and xe2x80x9cindustrialxe2x80x9d gas turbines take in wet air, the temperature and moisture conditions of which are such that the filters on the intake side of these machines may be subjected to iceing when the ambient temperature is below a certain value. Such iceing, which generally affects only the first filtration stage if several stages are used, may have very substantial negative effects and compromise the power consumption and reliability of the installation. Thus, depending on the design and structure of a given filter, on its mode of operation and on its degree of soiling at a given moment, the effects of the iceing may lead to an excessive and inadmissible rise in the pressure drop across the intake filters.
The technical risks run by the compression machine located downstream and which are specific to this situation are high and, in consequence, the manufacturers of these machines frequently insist or recommend that the intake air be heated upstream of the filters so as to avoid any risk associated with possible iceing of the filters. In the particular case of xe2x80x9cindustrialxe2x80x9d gas turbines, heating is dictated when the temperature of the external air drops below +5xc2x0 or even +8xc2x0 C. and/or according to the relative humidity of the air. In the case of compressors for air that is to be distilled, the recommended heating threshold is generally close to these values. This heating is generally dictated irrespective of the type of filter installed on the intake side of these machines.
Now, heating the air results in a reduction in the density of the air, and therefore in a drop in compression or installation performance. This drop in performance associated with the heating may reach 3 to 7%, and this may be for a period of several months per year, which is extremely costly for the operator of the compression machine.
An object of the invention is to make it possible to dispense with the heating of the air associated with the problems of iceing of the filters, while at the same time providing the compression machine with effective protection against soiling.
To this end, a subject of the invention is a method of the aforementioned type for compressing atmospheric air, characterized in that:
for the or the first filtration stage, use is made of bag-filter elements, the filter medium of which is made up of synthetic fibres with a density which increases in the direction of flow of the air through this medium; and
the air is introduced into the said bag-filter elements at its natural temperature.
Other subjects of the invention are:
an apparatus for compressing atmospheric air, of the type comprising a centrifugal or axial compressor, possibly a droplet separator, and a filtration assembly comprising at least one filtration stage, characterized in that the or the first filtration stage comprises bag-filter elements, the filter medium of which is made up of synthetic fibres with a density which increases in the direction of flow of the air through this medium;
an installation for distilling air, characterized in that it comprises an apparatus for compressing atmospheric air as defined hereinabove, supplying an air distillation apparatus; and
a gas turbine installation, characterized in that it comprises an apparatus for compressing atmospheric air as defined hereinabove, supplying a gas turbine of the industrial type.